In recent years, with the advancement of personal computers, and especially wide-screen liquid crystal televisions, the demand for liquid crystal displays (LCDs), in particular for liquid crystal color displays has tended to increase. Further, due to the demand for much higher image quality, the popularization of organic EL displays has been eagerly awaited.
Meanwhile, the demand for solid-state image pickup devices such as CCD image sensors has been significantly growing in accordance with the popularization of digital cameras, camera-equipped mobile phones, etc. Color filters have been used as a key device of such displays or optical devices, and the demand for cost reduction of color filters has been increasing in conjunction with the demand for higher image quality. A color filter generally has a colored pattern of three primary colors, red (R), green (G), and blue (B), and serves to color the transmitting light or separate it into the three primary colors, in image display devices or solid-state image pickup devices.
Coloring agents used in the color filter are commonly required to have the following characteristics. That is, they are required to have preferred light absorption characteristics in view of color reproducibility, to exhibit no occurrence of optical disturbance such as light scattering responsible for lowering of contrast in liquid crystal displays or non-uniformity of an optical density responsible for color unevenness or rough feeling in solid-state image pickup devices, to have favorable fastness for the environmental conditions under which they are used, such as, for example, heat resistance, light resistance and resistance to moist heat, and to provide a large molar absorption coefficient and the possibility of thickness reduction.
A pigment dispersion method has been used as one of the methods for producing color filters for use in solid-state image pickup devices, liquid crystal displays, organic EL displays or the like. In the pigment dispersion method, a color filter produced in accordance with a photolithographic method or an ink-jet method is stable against light and heat due to the use of pigments.
In a case in which the color filter is produced by the photolithographic method, a radiation-sensitive composition is coated on a substrate by a spin coater, a slit coater, a roll coater, or the like, and then dried to thereby form a coating film. Then, colored pixels are obtained by patternwise-exposing and developing the thus-formed coating film. The color filter can be prepared by repeating such an operation several times in accordance with a desired number of hues. Since patterning is performed by means of light, positional accuracy can be sufficiently secured, and accordingly, this method has widely been used as a method suitable for producing a high-definition color filter in a large screen for use in liquid crystal displays, organic EL displays, or the like.
In a color filter for use in solid-state image pickup devices, minute patterns are formed by exposure through a mask with minute patterns, development with an alkaline liquid, and dissolution of the unexposed parts in an alkaline developer solution, but it is difficult to control the solubility (developability) of the exposed parts and the unexposed parts.
Further higher definition of color filters for use in solid-state image pickup devices has been recently demanded, but further improvement of resolution has been difficult with the conventional pigment dispersion method, due to the problem of color unevenness or the like resulting from coarse particles of pigments, and application of a photolithographic method using a pigment dispersion method for enhancement of definition is becoming increasingly difficult.
Meanwhile, also in liquid crystal displays, organic EL displays or the like, the color filter produced by a photolithographic method using a pigment dispersion method is excellent in light resistance and heat resistance, but has significant problems of lowered contrast or increased haze due to light scattering by pigment particles.
Further, the photolithographic method has the problem of high-production costs due to the requirement to repeat respective processes of coating, drying, patternwise exposure and development of a radiation-sensitive composition several times in accordance with a desired number of hues as described above, and the problem of a decreased production yield due to repetition of the same processes. In particular, due to the increased demand for cost reduction of liquid crystal displays, the demand for cost reduction of the color filters, which account for a large proportion of the cost, has increased, so there has been an increasing demand for a color filter production method with higher productivity.
As a production method of a color filter intended to solve the problems of a photolithographic method, there has been proposed a method of forming a colored layer (color pixels) by jetting a colored ink with an ink-jet method (see Japanese Patent Application Laid-Open (JP-A) No. 59-75205 and JP-A No. 2004-339332).
The ink-jet method is a recording method of obtaining letters or images by directly jetting ink droplets from very minute nozzles to attach them onto a recording member. The ink-jet method has advantages in that a color filter with a large area can be produced with high productivity by sequential movement of ink-jet heads, and the production can be achieved with low noise and good operability. An ink-jet ink prepared using a pigment dispersion method is used in the production of a color filter in accordance with such an ink-jet method. As the ink-jet ink prepared using a pigment dispersion method, there has been proposed, for example, an ink-jet ink for a color filter, containing a binder component, a pigment, and a solvent having a boiling point of 180° C. to 260° C. and a vapor pressure at room temperature of 0.5 mmHg or less (JP-A No. 2002-201387).
When an ink-jet ink prepared using a pigment dispersion method is used in the production of a color filter, there are frequent occurrences of nozzle clogging due to aggregation of a pigment, so it is undesirable in terms of jetting stability. Further, the aggregated pigment leads to deterioration of the recovery function of the ink jetting state by a jetting recovery operation such as wiping or purging. In addition, when wiping is carried out, scraping of a nozzle face with the aggregated pigment may cause deflection of the jetting direction of ink.
When a dye is used in place of the pigment dispersion method, color filters for solid-state image pickup devices are expected to achieve high resolution by solving the problems of color unevenness and rough feeling, whereas liquid crystal displays or organic EL displays are expected to achieve improvements in optical properties such as contrast and haze. In addition, the ink-jet method using a dye generally has high jetting stability and is expected to achieve easy recovery of an ink jetting state by wiping or purging even when there is nozzle clogging associated with an increased ink viscosity or the like.
Based on the above-mentioned reasons, the use of a dye as a coloring agent has been examined (for example, see JP-A No. 2005-316012). However, a dye-containing colored curable composition has other problems as follows.
(1) Dyes are generally poor in light resistance and heat resistance as compared to pigments. In particular, there is a problem in that optical properties are changed due to a high-temperature process when forming a film of indium tin oxide (ITO) widely used as an electrode for liquid crystal displays or the like.
(2) Dyes tend to inhibit a radical polymerization reaction, so there is difficulty in designing of a colored curable composition, for a system where radical polymerization is used as a curing means.
Particularly, with regard to the photolithographic method,
(3) Conventional dyes exhibit low solubility in an alkaline aqueous solution or organic solvent (hereinafter, also referred to simply as “solvent”), and thus, it is difficult to obtain a colored curable composition with a desired spectrum.
(4) Dyes often exhibit interaction with other components in the colored curable composition, so it is difficult to control the solubility (developability) of the exposed parts and the unexposed parts.
(5) When a molar absorption coefficient (ε) of the dye is low, a large amount of the dye needs to be added. Therefore, the amount of other components such as a polymerizable compound (monomer), a binder and a photopolymerization initiator in the colored curable composition has to be relatively decreased, thereby reducing the curability, post-curing heat resistance, and developability of the composition.
Because of these problems, it has been difficult hitherto to form a colored pattern for high-definition color filters, which is composed of a fine thin film and has excellent fastness, using a dye. In addition, with regard to color filters for solid-state image pickup devices, a colored layer is required to be formed of a thin film having a thickness of 1 μm or less. Therefore, in order to achieve desired absorption, a large amount of the colorant needs to be added to the curable composition, consequently resulting in the aforementioned problems.
Further, with regard to a colored curable composition containing a dye, it has been pointed out that, when a heating treatment is applied after the formation of a film, color transfer readily occurs between adjacent differently colored patterns or between stacked and overlapped layers. In addition to the color transfer, pattern peeling readily takes place in a low-exposure dose region due to the decreased sensitivity, and a desired shape or color density cannot be obtained due to thermal sagging, elution upon development, or the like which is caused by the decrease in the retative amount of photosensitive components contributing to photolithographic properties.
As approaches to solve these problems, there have been conventionally proposed a variety of methods involving selecting the kind of initiators, increasing an addition amount of initiators, or the like (for example, see JP-A No. 2005-316012). Further, there has been disclosed a method of producing a color filter wherein a colored pattern is formed, and then polymerization is carried out in an elevated exposure temperature state by irradiating light to the colored pattern while heating a substrate, thus increasing a polymerization rate of the system (for example, see Japanese Patent No. 3309514). In addition, there has been disclosed a method of producing a color filter wherein light irradiation is carried out between a development treatment and a heating treatment, thereby preventing shape deformation of the color filter (for example, see JP-A No. 2006-258916).